This invention relates to disk drives in general and more particularly to an isolation grommet which supports a component of the disk drive in vibratory isolation from the supporting structure of the grommet.
The basic configuration of the isolation grommet of the prior art is shown FIG. 1 at 10. Such a grommet is generally made of an elastomeric material such as rubber so that any vibration of the supporting structure damps within the grommet itself rather than being transmitted to the component to be supported in isolation. Grommet 10 comprises a relatively thick and stiff outer portion 14 which is separated from inner portion 11 containing a concentric bore 12 by a relatively thin intermediate portion 13. Grommet 10 may be attached to the supporting structure (a portion of which is illustrated at 16) by fasteners 15. The component 18 to be supported in isolation may be part of a disk drive and has an attachment ferrule 17 connected thereto. Ferrule 17 snugly fits within bore 12 of inner grommet portion 11. The centerline of ferrule 17 coincides with the respective centerlines of bore 12 and grommet 10 to define a symmetrical grommet as will be subsequently discussed. Vibration of supporting structure 16 of grommet 10 is absorbed or damped within inner portion 13 of the grommet rather than being transmitted to component 18. Component 18 may be the sealed housing which contains the disks and positioner of a disk drive. It may be supported within an outer supporting structure or housing by four grommets.
The problems incurred when using symmetrical grommets can be best demonstrated by reference to FIGS. 2-4. Since the isolation grommet's function may be analogized as a spring, its operation may be represented by a stiffness or force vs. deflection curve such as that illustrated in FIG. 2. The stiffness of an elastomeric isolation grommet is a function of the grommet shape, the modulus of elasticity of the grommet material and the amount of deformation. For most grommet shapes and elastomer materials the force vs. deflection curve will be nonlinear over some or the entire portion of the curve. Thus, the grommet may have a break point in the curve where its Hookes constant changes from one value to another. Such a point is illustrated in FIG. 2 at d. The amount of deformation or deflection that will be produced by a given force acting on a linear isolation grommet of a specified shape and modulus of elasticity may be represented by the following equation: EQU f=kx,
where f is the force applied to the grommet, k is the value of Hookes constant and x is the amount of deflection. Since the value of Hookes constant or k changes at point d of the curve as shown in FIG. 2, the amount of deformation produced by a given force varies according to the portion of the curve in which the grommet is operating.
It is often desirable to control or limit the region of the force-deflection curve in which the isolation grommet is to operate during specified shock and vibration loads to the linear portion of the curve in order to accurately predict and design for the response of the grommet to shock and vibration input changes. The symmetrical isolation grommets often operate in the undesirable nonlinear portion of the stiffness curve as can be seen with reference to FIGS. 3 and 4. If dynamic forces are applied to a symmetrical isolation grommet already under a static load, the grommet often deflected such that it would not properly isolate against the dynamic forces.
FIG. 3 is a simplified sectional view of symmetrical isolation grommet in an unloaded condition. The attachment ferrule is schematically indicated at 17 and is snugly secured within bore 12 of mid-section 11 of the grommet. Bore 12 is symmetrically located on the centerline g--g of the grommet such that the centerline of the ferrule f--f coincides with the centerline of the grommet g--g. FIG. 4 shows the condition of the grommet when a load designated as L is imposed upon the grommet. Such load may be the static weight of the component to be isolated by the grommet. Under load, the flexible intermediate region 13 of the grommet deforms allowing the thick central portion 11 and the lower edge 14 of the grommet to contact or snub. Since any further deformation of the grommet would now include the thick region 14, this effectively changes the stiffness or Hookes constant of the grommet such that the grommet no longer operates in the linear portion of the curve. When the operating portion of the curve is changed, the response of the grommet to shock and vibration inputs also changes. As previously mentioned, the problem with the prior symmetrical grommets was that in use the weight of the object supported deformed the grommet causing the undesirable snubbing situation just described. More particularly, the problems caused by the preload deformation due to the weight of the object to be supported were:
(1) a reduction of the sway or damping space since the object supported is already at a limit of its traversal; and PA1 (2) a different response to shock and vibration inputs than may have been desired or designed for since additional deformation may cause the object to move into the nonlinear portion of the stiffness curve.